I know you answered your own question, but that would be a great feature. The problem is, when you go to invoke Siri by saying ‘Hey Siri’, and then the Siri display shows, How would the phone be answered When she’s listening for your command? And by that time, the caller would be sent to voicemail because of the time lapse. It’s just too difficult of a measure to implement when you look at the time measure, as the screen has to alter to hear the command, And then recognize to have the call answered Successfully, assuming Siri understood what you said initially the first time accurately.
Plus, then factor in all the ambient noise around you, it probably Would be too distracting to make it actually work, even though I do think it is possible, Apple probably just wouldn’t choose that route given the other circumstance as I mentioned above.